1. Field of the Invention
The present invention relates to a semiconductor memory device and operating method thereof. More particularly, the present invention relates to a one-time programmable read only memory and operating method thereof.
2. Description of the Related Art
As the feature size of the semiconductor devices reaches the deep sub-micron range, the dimension of each device is correspondingly reduced. For a memory device, the reduction in dimension implies the area for accommodating of each memory cell is decreased. However, as the quantity of data that needs to be processed and stored inside an electronic communication product (for example, a computer, a mobile phone, a digital camera or a personal digital assistant) continues to increase, the memory inside the electronic product must store an ever-increasing quantity of data. Hence, there is a demand for methods capable of fabricating smaller memory with a larger memory storage capacity. Currently, one major goal of the electronic industry is to fabricate semiconductor devices having a smaller size, a higher level of integration and an improved quality.
According to the difference in reading/writing function, memory can be categorized into two major types: read only memory (ROM) and random access memory (RAM).
The read only memory (ROM) can be further categorized into erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), mask read only memory (mask ROM) and one-time programmable read only memory (OTPROM).
Although EPROM and EEPROM has both a write-in and an erase function and hence has a wider range of actual applications, these memories also have a more complicated fabrication process and a higher cost of production.
On the other hand, although a mask ROM is simple and inexpensive to fabricate, a photomask is required to define the write-in data. Hence, the mask ROM has some limitations in real applications.
For a one-time programmable read only memory (OTPROM), data can be written into the memory after leaving the factory. In other words, the operating environment of the memory can be programmed through a write-in operation carried out by the users in their premises. Thus, the OTPROM is more convenient to use compared with the mask ROM.
The random access memory (RAM) can be categorized into static random access memory (SRAM) and dynamic random access memory (DRAM).
According to the design method, a DRAM cell generally includes a transistor and a capacitor. A bit of digital data is stored inside each DRAM cell according to the charging state of the capacitor. In other words, the presence or absence of electric charges inside the capacitor on the substrate is used to represent a ‘0’ or a ‘1’ logic state in a binary storage format. However, due to the leakage current in the device, the data stored inside the DRAM cells need to be constantly refreshed leading to power wastage. Moreover, the stored data is lost forever once the power supplying the DRAM is removed.